German Patent No. 31 17 341 describes an arrangement in which an ink application cylinder has effectively the same diameter as the plate cylinder. The plate cylinder may carry one or more printing plates thereon. The ink application cylinder, the diameter of which corresponds to the effective working diameter of the plate cylinder with the printing plates is driven to have the same circumferential speed as the plate cylinder and, in the engagement zone between plate cylinder and ink application roller, it rotates in the same direction. The ink application cylinder has a yielding surface.
The yielding surface of the ink application cylinder causes slippage and rubbing between the ink application cylinder and the plate cylinder, due to the compression of the yielding surface of the ink application cylinder as the consequence of engagement pressure between the two cylinders. This slippage and rubbing causes excessive wear on the printing plates, heats the cylinders, and also causes problems in connection with supply of damping fluid, typically water. The heating leads to expansion of the volume of the working surface of the ink application cylinder, which then changes the engagement relationships between the engaged cylinders, further increasing the rubbing effect. More damping fluid is emulsified in the ink due to the slippage and rubbing than would be the case if there were no slippage. This damping fluid then is no longer available for application to the surface of the printing plate in the region where printing is not to be effected. The result is scumming or tinting of the printing substrate. Increased supply of damping fluid counteracts such scumming. The ability of most inks to emulsify damping fluid has a limit, however, and thus, if too much damping fluid is applied, damping or water marks may occur on the substrate. Additionally, the viscosity or flowability of many inks is undesirably affected if the proportion of water emulsified therein is too high.
U.S. Pat. No. 2,036,835, to which German Patent No. 625,327 corresponds, disclose that slippage or rubbing occurs between the plate cylinder and the blanket cylinder of an offset printing machine if both cylinders have exactly the same working diameters. To avoid such slippage, it has been proposed to slightly increase the diameter of the plate cylinder and decrease the diameter of the blanket cylinder. When using incompressible blankets, this opposite relationship then avoids slippage and rubbing. Rubber blankets which are incompressible deform, however, so that, upon compression of the rubber blanket by the plate cylinder, a bulge will be formed.
It has been found that changing the diameters of the plate and rubber blanket cylinder is not a suitable solution when using compressive or compressible blankets on the blanket cylinder. Compressible blankets decrease the volume due to compression by the plate cylinder. The change in the diameters of the respective cylinders does not remove the rubbing or slippage between the cylinders.
Using excess damping fluid, regardless of the diametrical relationship of the blanket cylinder and the plate cylinder, raises special problems when inkers are used which include an anilox cylinder to supply ink. Returned or fed-back ink-damping fluid emulsions hardly evaporate from an anilox cylinder. There is, therefore, only a very narrow range in which just sufficient, but not excessive damping fluid can be supplied. Adjusting the quantity of supply of damping fluid within this narrow range is difficult and expensive. It has been found, further, that the proportion of damping fluid emulsified within the ink increases as the slippage or rubbing increases.
Changing the relative diameters of the plate cylinder and an ink application cylinder in opposite directions is often not possible since the working diameter of the plate cylinder is determined with reference to the blanket cylinder. Driving the ink application cylinder with a speed which differs from that of the plate cylinder is likewise not possible since, otherwise, striping or ghost patterning may occur. Thus, any changes in diameter to provide for a relative difference between plate cylinder diameter and ink application roller diameter must be accepted by the ink application roller. Consequently, the spacing of the shaft centers of the plate cylinder to the ink application cylinder will change. The shafts, however, carry gears of equal size in order to obtain the appropriate 1:1 transmission ratio. It is thus possible to compensate for changes in axial spacing by shifting the gear profiles only within very small dimensions.
The discussion in the aformentioned U.S. Pat. No. 2,036,835 with respect to relative diametric relationships of the blanket cylinder and the plate cylinder is restricted specifically to these two cylinders, and what could happen if the ink application roller or cylinder has a compressible surface is not disclosed.